The advent of hydrostatic transmissions for use in construction vehicles and the like has given rise to the need for assuring that the transmission cannot be driven upon starting of an engine and with a control valve therefor placed in its "run" position. In order to satisfy this need, various types of mechanical linkages have been proposed for connection with the control valve to make certain that the control valve is reset to a vent position prior to placing the transmission in its running mode of operation and upon starting of the engine. Such linkages are somewhat complex and require finite setting to achieve such a reset function. In addition, the rather bulky control valve and associated hardware of the transmission control group are prone to leakage and cannot be compactly arranged for ready servicing within the vehicle. A typical transmission of the above type is disclosed in U.S. Pat. No. 3,952,514, issued to Cyril W. Habiger on Aug. 27, 1976, such patent being assigned to the assignee of this application.